Amount of stone (3)
I believe the answer is:
ton
'amount' is the definition.
(ton is a kind of amount)
'of stone' is the wordplay.
'of' indicates the answer is hidden within the clue.
'TON' is hidden within 'stone'.
(Other definitions for ton that I've seen before include "Weight - fashion (French)" , "100 mph, runs, etc" , "A slang 100mph - heavy" , "Two thousand, two hundred and forty lbs avoirdupois" , "Heavyweight" .)